The production of extremely short pulses from lasers is of considerable interest and, accordingly, techniques have been devised which permit lasers to produce pulses of short temporal duration. These lasers may be, for example, dye, color center, gas, solid state or semiconductor lasers and promise to be useful in such commercial applications as time division multiplexing communications systems in which several bit streams of short duration pulses are interleaved. Of course, the pulse shape, as well as the pulse duration, are of interest.
The techniques that have been developed to produce short pulses fall into two general categories which will be discussed separately. First, there is modelocking. Exemplary of this technique is the article in Applied Physics Letters, 38, pp. 671-672, May 1, 1981 by Fork et al. Fork et al describe a passive modelocked laser which uses the interaction of two oppositely moving pulses in a thin saturable absorber to produce short pulses. The technique is more precisely referred to as colliding pulse modelocking. Pulses of a duration shorter than 65 fsec were obtained. The saturable absorber was 3-3'-diethyloxadicarbocyanine iodide (Rhodamine 6G) in a solvent of ethylene glycol. The saturable gain medium was a dye laser that was pumped with a cw argon laser. Two pulses move in opposite directions around the ring configuration, collide in the saturable absorber and couple energy between the pulses in such a way that the pulses have equal energies. The interaction of the two pulses produce a transient grating which, for example, acts to shorten the pulses. It should be noted that nonring embodiments are also discussed. The two essential elements of the modelocking technique are the presence of saturable absorption and saturable gain.
Second, there is soliton type pulse shaping. Exemplary of this technique is the article in Optics Letters, 9, pp. 13-15, January 1984 by Mollenauer et al which describes what the authors term a soliton laser. According to one common definition a soliton is a pulse or a wave which moves without either dispersion, gain or loss of energy. A more generally accepted definition is that a soliton is a pulse whose shape periodically repeats during propagation. The soliton laser described optically couples two resonators. The first resonator generates a short pulse in a, e.g., color center modelocked laser. The output pulse from the color center laser resonator goes to a second resonator which has a single mode, polarization preserving optical fiber after which the pulse is then reinjected into the first resonator. The optical fiber has characteristics such that the group velocity dispersion and self phase modulation are of opposite signs. The group velocity dispersion is defined as the partial derivative of the group velocity with respect to wavelength. The self phase modulation is the result of a nonlinear refractive index with respect to light intensity. Thus, for example, frequencies in the leading half of the pulse are lowered while frequencies in the trailing half are raised.
This is better understood from the following considerations, if the group velocity dispersion is negative, i.e., anomalous, the leading or lower frequency portion of the pulse is retarded while the trailing or higher frequency portion of the pulse is hastened. The net result is that both pulse compression and shaping occur.
The observation of pulse narrowing and solitons in optical fibers was reported earlier in Physical Review Letters, 45, pp. 1095-1098, Sept. 20, 1980 by Mollenauer et al. It should be noted that the term soliton as it is commonly used also includes higher order solitons. Such solitons may actually contain what appear to be two or more pulses, the number depending upon the order, with the energy being periodically transferred between the pulses. However, any given pulse shape will be periodically repeated during propagation. The higher order solitons are generated as the peak pulse power is increased. Of course, there is also a minimum power level which must be exceeded for solitons to be generated. This technique requires anomalous dispersion to balance the self phase modulation. Mollenauer et al reported significant narrowing of a pulse from a color center laser.
It should also be pointed out that although the anomalous dispersion of a fiber was used to generate soliton shaping, other methods of obtaining negative dispersion are known to those skilled in the art. For example, the article in Optics Letters, 9, pp. 150-152, May 1984 by Fork et al describes using two pairs of prisms, i.e., four prisms to obtain negative dispersion. The prisms are identical and used at the minimum deviation angle and Brewster's angle incidence at each surface. The arrangement disclosed in advantageous because the negative group velocity dispersion is adjustable. The dispersion constant is determined by the second derivative of the optical path length with respect to the wavelength.
This prism configuration was used in a colliding pulse modelocked laser and pulse shortening was observed. The authors speculated that still shorter pulses might be obtained by adjusting the amounts of self phase modulation and negative dispersion.
Consideration of the above lasers shows that there are essentially four elements which are used to control the pulse duration: (1) saturable absorption; (2) saturable gain; (3) group velocity dispersion; and (4) self phase modulation. Typically, all four are always within a single laser and are therefore present and affect the laser operation to some degree. However, the lasers described are not optimized with respect to balancing all four effects as the first two effects are coupled with each other and the last two effects are also coupled with each other. For example, optimal saturable absorption and group velocity dispersion are not obtained simultaneously. Therefore, optimization has not occurred.
Theoretical studies have been done in which all four effects are simultaneously treated mathematically. For example, Optics Letters, 9, pp. 156-158, May 1984 describes one such study of passively mode locked lasers. While all effects are contained mathematically in the described study to some degree, there is no teaching of how to implement such a laser and there is no analysis of the effect of deep saturation of the absorber.